The present invention generally relates to electrochemical cells and, more particularly, to a low profile seal assembly for sealing the open end of a cell container and having a pressure release mechanism for venting when exposed to excessive pressure.
Conventional alkaline electrochemical cells generally include a steel cylindrical can having a positive electrode, referred to as the cathode, which comprises manganese dioxide as the active material. The electrochemical cell also includes a negative electrode, referred to as the anode, which comprises zinc powder as the active material. The cathode is typically formed against the interior surface of the steel can, while the anode is generally centrally disposed in the can. Alternately, in jelly-roll cells, the anode and cathode are spirally wound. A separator is located between the anode and the cathode, and an alkaline electrolyte solution simultaneously contacts the anode, the cathode, and the separator. A conductive current collector is commonly inserted into the anode active material, and a seal assembly, which includes a seal member, provides closure to the open end of the steel can to seal the active electrochemical materials in the sealed volume of the can.
Cylindrical alkaline cells are commonly sealed closed by placing an annular nylon seal above a bead in the open end of the steel can and crimping the upper end of the can inwardly and over the outer periphery of the seal to compress the seal against the bead. However, electrochemical cells commonly employ electrochemically active materials, such as zinc, which generate hydrogen gas during storage and sometimes during or following service use. When the can is sealed closed, excessive build-up of high pressure gases within the sealed can may cause damage to the cell and/or the device in which the cell is employed.
One approach to avoiding a potentially excessive build-up of pressure in a cell has been to employ a resealable valve system that periodically releases excessive gas pressure from within the active cell volume. However, the continued periodic release of gas pressure may, in some situations, permit the release of electrolyte solution containing salts or other particulate matter, which may foul the resealable valve, and generally requires additional costly components. Another approach to avoiding excessive build-up of pressure involves employing a sealed membrane that is intended to blowout when exposed to excessive pressure either by puncture or rupture of the membrane itself. Puncture mechanisms such as a spiked member may be employed to punch a hole in the sealed membrane once the pressure reaches a predetermined amount. Alternately, a rupture mechanism may be employed in the form of a thin membrane which ruptures when the internal pressure of the cell becomes too great.
Other approaches to venting excessive pressure have included the use of a vent formed in the seal which is intended to rupture upon experiencing an excessive pressure build-up in the cell. For example, U.S. Pat. No. 5,667,912 discloses a current collector assembly having a low profile seal with a thinned portion which is intended to shear when the internal pressure exceeds a predetermined pressure. The disclosure of U.S. Pat. No. 5,667,912 is hereby incorporated by reference.
When the seal has a thinned portion or other stress concentration vent formed axisymmetrically about a complete 360 degree rotation of the cell's central longitudinal axis, a potential problem may exist in that the vent, when completely activated, may force the vented diaphragm portion of the seal outward throughout the full 360 degree rotation. If a cover is located close enough to the venting seal, it is possible that the vented seal diaphragm may make contact with the surface of the cover with such force that a pressure-holding seal is made. The resealing of the cell during a 360 degree vent condition may result in the build-up of an excessive pressure beyond which the cell was intended to operate. In this circumstance and when excessive pressure is experienced, the crimp formed at the rim of the can may be forced to release and thereby open the cell's container. Accordingly, it is therefore desirable to provide for an electrochemical cell having a low profile seal assembly that effectively vents, when necessary, and prevents resealing of the seal member.